The invention relates to a method of determining the optical quality of and for detecting faults in flat glass.
A method is known for determining the optical quality of flat glass, especially of float glass, where in a video camera is arranged to monitor illumination through glass or a transparent sheet. In the process, the video camera receives signals according to the quality of the glass. These signals will subsequently be evaluated.
In the production of glass or other transparent materials, it may happen that lines in the longitudinal direction and threads and faults with slight optical deformation are generated which have an optical deflection only in the transverse direction of the sheet. Such faults cannot be detected by conventional dark-field techniques and other methods which pick up the deflection in the longitudinal direction only.
In the following, a novel method of detecting glass faults will be presented. The method detect""s the deflection of a glass fault in the transverse direction of the glass sheet and is particularly suited for the detection of lines, threads and faults with slight optical deformation (drips).
The inventive method is characterized in that at least one illuminating device comprising a pattern of adjacent partial portions alternately different at least in color and/or in intensity is used, an observation spot of the video camera picks up the pattern, two video signals U1,U2 are assigned to the signal of the pattern, and a change of the intensity of the video signals U1,U2 is used for evaluating the quality of the glass and the sheet, respectively.
According to an advantageous embodiment of the invention, use is made of a two-colored pattern comprising partial portions of alternating colors, and the video camera includes a color chip, with the video signals assigned respectively to one color.
Suitably, the width of the partial portions is slightly larger or smaller than the lateral length of the observation spot. Only the evaluation is changed in the given case.
In an alternative advantageous embodiment of the invention, the pattern comprises partial portions of alternating bright and dark zones. Advantageously, the partial portions are illuminated and obscured cyclically in predetermined phases.
Suitably, a second, identical illuminating device is used whose pattern is arranged at a displacement by half of the width of a partial portion relative to the pattern of the first illuminating device. In this manner, dead zones are excluded.
Advantageously, the difference between the two video signals U1,U2 is used as a measure of the deflection caused by the fault in the glass.
According to a particular embodiment, the relation   Upos  =                    U        1            -              U        2                            U        1            +              U        2            
is used as a measure of the deflection caused by the fault.
Advantageously, for measuring the core of the fault in the glass, use is made of a deviation from the maximum value of the addition of U1 and U2, i.e. from Uh=U1+U2.